Electrochemical oxygen reduction/evolution reactions (ORR/OER) play a crucial role in many decisive energy conversion and industrial production reactions in both acidic and alkaline electrolytes such as acidic/alkaline fuel cells, metal-air batteries, water-splitting and chlor-alkali electrolysers. The coupled multi-electron and multi-proton O2 reduction/evolution processes involve several intermediates and reaction barriers, which makes the ORR/OER inherently kinetically sluggish and thus poses great challenges to the development of highly active bi-functional oxygen catalysts.
To date, Pt- and Ir-based compounds are recognized as the most efficient ORR and OER catalysts, respectively, but their broad application is significantly limited by the concomitant affordability and scarcity of the noble metals. Additionally, noble metal-based catalysts suffer from poor durability caused by different mechanisms including dissolution, sintering, ripening and leaching as well as susceptibility to crossover effect induced deactivation in case of Pt/C. Therefore, cost-effective highly efficient and stable ORR/OER materials as alternatives to noble metal catalysts are urgently required.
Among various catalysts developed so far, heteroatom-doped carbon materials exhibit a great potential as candidates for exerting ORR/OER owing to their unique electronic and structural features and robustness as matrix in different electrolytes. Although great advances have been achieved, most of the non-metal element doped carbon materials as metal-free catalysts still show inferior intrinsic ORR/OER activities compared to noble metal-based ones. In principle, a favored ORR/OER catalyst should be capable to deliver a sufficiently high reaction current at low overpotential to warrant efficient power output and energy conversion efficiency. Within this context, it seems that efficient ORR/OER carbon materials should feature good conductivity for charge transfer, abundant accessible active sites for implementing reaction, and a suitable porous structure for mass transport. Porosity is thereby proposed as a pivotal factor that affects all three aspects, and appropriate porous structure engineering can render a large specific surface area and facilitate harness of active sites, which can affect ORR/OER electroactivities.
To achieve sufficient porosity, various templates or pore-generators, such as silica (Liang, H.-W. et al., Nature Commun. 5, 4973, 2014, Liang, J. et al., Angew. Chem. Int. Ed. 51, 11496-11500, 2012), porous alumina (Liang, C. et al., Angew. Chem. Int. Ed. 47, 3696-3717, 2008), polystyrene (Liang, J. et al., Adv. Mater. 26, 6074-6079, 2014) and cellulose (He, W. et al., Angew. Chem. Int. Ed. 53, 9503-9507, 2014) have been adopted with the resulting porous carbon materials exhibiting increased performances. Nevertheless, additional activation steps such as by NH3 or KOH treatment are often required to fulfill the desired hierarchical nanoarchitectures (He, W. et al., Angew. Chem. Int. Ed. 53, 9503-9507, 2014, Liang, H.-W. et al., Nature Commun. 5, 4973, 2014). Moreover, the deliberately introduced rigid templates have to be removed by cumbersome post treatments. For example, silica needs toxic HF solution or concentrated alkaline reagents at elevated temperatures, whilst polystyrene and cellulose templates have to be removed by lengthy immersion in organic solvents (Liang, J. et al., Angew. Chem. Int. Ed. 51, 11496-11500, 2012, Liang, J. et al., Adv. Mater. 26, 6074-6079, 2014). Those additional steps limit the potential scale-up of these processes and can be associated with the leaching of relevant active species. Respective prior art methods with such additional steps are referenced herein as “two-step” processes.
Therefore, there remains a strong need for a fast and economically-efficient synthesis approach for preparing hierarchically porous doped carbon materials with appropriate properties and sufficient catalytic performance such as for ORR/OER reactions. In particular such method should be suitable for industrial scale production.